<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Media</title>
    <style>
        .container{
            max-width: 750px;
            padding-top: 20px;
            margin: auto;
        }
        .box {
            padding: 20px;
            background: #eee;
            font-size: 2rem;
        }
        @media (max-width: 500px) {
            .box {
                font-size: 1.5rem;
                padding: 10px;
                text-align: center;
            }
        }
    </style>
</head>
<body>

    <div class="container">
        <div class="box">
            Это урок про адаптивную верстку.
        </div>

    </div>

</body>
</html>